1. Field of the Invention
The present invention relates to a solid-state image pickup device constituted by semiconductor elements, and particularly to an X-Y address type solid-state image pickup device manufactured by a CMOS process.
2. Description of the Related Art
A solid-state image pickup device is roughly classified into a CCD (Charge Coupled Device) solid-state image pickup device constituted by a charge transfer type image sensor, and an X-Y address type solid-state image pickup device in which an image sensor is constituted by, for example, CMOS (Complementary Metal Oxide Semiconductor) transistors. The X-Y address type solid-state image pickup device using a CMOS image sensor (hereinafter suitably abbreviated to a CMOS image sensor) can be manufactured by the same technique as a manufacturing process of a MOSFET, and it is driven by a single power source and has low electric power consumption, and further, various signal processing circuits can be mounted on the same chip. Thus, the CMOS image sensor is regarded as promising in substitution for the CCD solid-state image pickup device.
The CMOS image sensor includes a plurality of pixel regions defined by a plurality of vertical selection lines and horizontal selection lines in a matrix form, and a photoelectric transducer such as a photodiode is formed in each of the pixel regions. Light incident on a light receiving surface of each photoelectric transducer is subjected to photoelectric conversion, and an electric charge is stored in the transducer. The stored electric charge is converted into a voltage by a source follower amplifier provided in the pixel and is amplified, and the voltage is read out as image data of one pixel at a predetermined timing.
As a use mode of the CMOS image sensor, for example, in order to intentionally obtain a rough display resolution, there is a case of executing a processing to decrease the number of image data by carrying out an averaging processing of image data of neighborhood pixels. In the image averaging processing, analog image data of the respective pixels outputted from the CMOS image sensor are converted into digital data by using an A/D (Analog/Digital) converter, and after they are stored in a storage device once, the image data of the neighborhood pixels are averaged by using a logical processing.
However, in the above conventional image averaging processing, there arise a problem as follows: FIG. 11 schematically shows a pixel array of a light receiving surface of the CMOS image sensor. Each pixel is made to correspond to any one of the three primary colors R (Red), G (Green) and B (Blue). For example, in the pixel array shown in FIG. 11, at an odd line (in the horizontal direction), pixels are successively arranged from the left to the right like R, G, R, G, . . . , and at an even line, similarly, pixels are successively arranged like G, B, G, B, . . . . Accordingly, if an attempt is made to obtain one pixel for each of R, G and B by averaging image signals of R, G and B from 4×4=16 pixels surrounded by a broken line in the drawing, in the foregoing image averaging processing, the logical processing becomes extremely complicated, and further, the capacity of the storage device for storing the image data becomes large. Thus, if an attempt is made to incorporate an image averaging circuit in the same chip as the CMOS image sensor, there arises a problem that a chip area is largely increased, and manufacturing costs become high.